Speech transmission system



Jan. 27, 1942. w. v. LovELL SPEECH TRANSMISSION SYSTEM 2 Sheets-Sheet 1Filed Jan. 7, 1941 WEST 5 BJ Law 6 e2 7 2.a f@ mn OWU f j ,sA n

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SPEECH TRANSMISSION SYSTEM Filed Jan. '7; 1941 2 Sheets-Sheet 2/Nl/E/VTOR W V LOVE L L '9s/KMA A T TORNE V Patented Jan. 27, 1942UNITED STATES PATENT OFFICE SPEECH TRANSMISSION SYSTEM William v.Lovell, Sanford, Fia.. Application January 7, 1941, Serial No.`3'73,395

11 Claims.

The invention relates to a speech transmission system.

An object of the invention is to achieve increased efficiency in theoperation of certain'types of speech transmission systems.

A further object of the invention is the transmission of messages insuch a manner that intelligible reception of the message by unauthorizedpersons will be rendered very diflicult.

A more specic object of the invention is to decrease the idle time ofspeech transmission circuits. v

-It is recognized by transmission engineers that, during a so-calledconversation period on a four- Wire circuit, one side of the circuit isactually in use only about one-third of the time. `It has previouslybeen proposed to take advantage of this fact by associating a number ofsubscribers telephone circuits with a lesser number of oneway speechchannels and switching each different speech portion of vocal intervalduration occurring in the various' subscribers circuits to any one ofthe channels that happens to be idle at a particular moment. While suchsystems appear, in general, to provide worthwhile advantagesparticularly from an economic standpoint, there has been some objectionto the use of such an arrangement on high quality circuits due both tothe fact that distortion may creep in because of the switchingoperations and to lthe fact that small portions of the transmission maybe lost or delayed at times due to momentary lack of an idle channel. v

A feature of the present invention resides in means whereby only aselected circuit is switched about from idle channel to idle channel,thev other circuits operating over their individually assigned channels.

In accordance with a specific embodiment of the invention, a givennumber, such as three, west-east and a corresponding number, such asthree, east-west channels are provided for connecting, in theillustrative case, three subscriber stations at the west terminal of aspeech transmission system with three subscriber stations at the eastterminal for two-way conversations. y

wanted by the subscriber normally assigned thereto such subscriber willtake over the channel and the circuit of the fourth subscriber will beswitched onto the next idle channel. Should all channels be in use bythe preferred stations, interconnection of the fourth pair of subscriberstations will be delayed until an idle channel is available.

The above arrangement results in eiicient and economical operation ofthe system as six oneway channels provide the service ordinarilyprovided by eight such channels and yet there is no necessity ofswitching the high quality circuits from one channel to another withconsequent possibility 'of delayed transmission. It is contemplated thatthe switched circuit may, if desired, be vof the type ordinarily termeda low quality circuit wherein a small amount of `distortion oroccasional delays will not'be' objectionable. Such a circuit Vmay beutilized, for example, by Aattendants supervising the operation of thesystem. f

It will be apparent that a measure of privacy will be attained by the'arrangement described inasmuch as an unauthorized person listeningin onany of the channels between the terminals will hear snatches of twodifferent conversations interspaced one with the other; i. e., thenormal conversation of the channel and the switched conversation. Asomewhat higher degree of privacy will be attained, of course, withrespect to the switched conversation as snatches of this conversationwill appear on different n channels.

invention; and

Fig. 2 shows in detail the switching arrangement of one terminal of thesystem of Fig. 1.

Referring now to the drawings, there is il-l lustrated diagrammaticallyin Fig. 1, two terminals of a two-way telephone system which may be, forexample, a two-way radio or line Wire telephone system. Four subscribersets, Il, I2, I3 and I4, are shown at the west terminal each of which isprovided' with a suitable two-wayl subscribers telephone circuit;similarly, four subscriber sets, I5, I6, I1 and 2 I, are shown at theeast terminal, each of these sets also being provided with a suitabletwo-way subscribers telephone circuit.

The west terminal switching apparatus'is represented by box 22 and theeast terminal switching apparatus by box 23; these switchingarrangements will be described in detail subsequently in connection withFig. 2.

A west-east channel 24 and an east-west channel 25 are provided fortwo-way conversations between subscriber W-I and subscriber E--I, thearrangement being such that channel 24 is immediately available tosubscriber W-I whenever a speech portion of vocal interval duration isproduced by his subscriber set II and that channel 25 is immediatelyavailable to subscriber E--I whenever a speech portion of vocal intervalduration is produced by his subscriber set I5. Similarly, west-eastchannel 26 and east-west channel 21 are provided for two-wayconversations between subscribers W-2 and E2 and west-east channel 4|`and east-west channel 42 are `provided for two-way ,conversationsbetween vsubscribers W-3 and E--3.

It will be observed that conversations between subscribers W-I, W-2 andW'-3 and subscribers E-I, E-2 and E-S, respectively, are maintained on apreferred or high quality basis as the terminal circuits are notswitched from channel to channel. However, in accordance with thearrangement, a secondary service is provided for two-way conversationsbetween a fourth subscriber W--4 at the west terminal and la fourthsubscriber E-4 at the east terminal. It is contemplated that suchadditional facility might well be utilized by an attendant supervisingthe operation of the high quality circuits in which case slightdistortion or occasional delay would be particularly serious. Asindicated schematically, transmission from subscriber set I4 tosubscriber set 2l is achieved by switching the terminal circuits aboutamong channels 24, 26 and 4|, depending upon which is idle at any onemoment while transmission from subscriber set 2| to subscriber set I4 isachieved by similarly switching the terminal circuits about among`channels 25, 21 and 42. q

When voice frequency currents originating at subscriber set I4 areapplied to any of the westveast channels, a distinctive tone of lowfrequency produced by source 43 is applied to the same channel;similarly when voice frequency currents originating at subscriber set 2|are applied vto any of theeast-west channels a low frequency toneproduced by source 44 is applied tothe same channel. 'The purpose ofthese tones will be described subsequently in reference to Fig. 2.

The general switching arrangement contemplated is such that, so far astransmission from subscriber W4 to subscriber E-4 is concerned, channel24 is superior (i. e. preferred) to `channels 26 and 4| and channel 26is superior to channel 4I. That is, speech currents originating katsubscriber set I4 are always applied `to channel 24 so long as channel24 is not in use for transmission from W--I to lll-I. Should channel 24be in use, the speech currents originating at subscriber set I4 areapplied to channel 26 so long as channel 26 is not in use fortransmission from vW--2 to E-,2. Shculdlboth channels `24 and 26 be inuse, the speech currents of subscriber set `I4 4are then applied tochannel .4I so long as channel 4I is not in use for transmission fromW-B .to E-3.

Should all three channels be in use, transmission of the speech currentsof subscriber set I4 is delayed until one of the channels becomes idle.

In like manner with respect to transmission from E-4 to W-4, channel 25is superior to channels 21 and 42 and channel 21 is superior to channel42.

The term vocal interval as used herein means the duration of one or asuccession of syllables no one of which is separated from the precedingsyllable by an interval as great as the hangover time of the system,weakbeginnings and endings, i. e., portions not capable of causing relayoperation, being considered parts ofthe silent interval. Hang-over timeis a term commonly used in connection with systems includingvoiceoperated devices and means theperiod that a voice-operated deviceremains, due to its own characteristics, in operated position after theoperating impulse has been removed.

Referring now'to Fig. 2, a detailed showing is set forth of theswitching apparatus of the west terminal represented in Fig. 1 by box22.

Subscriber set II is coupled to transmitting line 14 and receiving line15 by transformer 16, network 11 serving to provide proper balance inthe well-known manner. In a similar manner, subscriber set I2 isconnected to transmitting line 8| and receiving line 82 throughtransformer 83, subscriber set I 3 is connected to transmitting line '84and receiving line 85 through transformer 06 and subscriber set I4 isconnected to transmitting line' 81 and receiving line IOI throughtransformer |02.

For purposes of further illustration of the invention, the operation ofthe circuits of Fig. 2 will now be described in detail. Let us assume,first, that subscriber W-I speaks, at which time speech energyoriginating at subscriber station II will be applied through transformer16 to transmitting line 14. As the speech energy is delayed somewhat inits passage through delay `device |03, let us rst see what operationsresult from passage of a part of the speech energy throughamplifier-detector |04.

The rectified and filtered speech currents emanating fromamplifier-detector |04 cause operation of relays |05 and |06. Operationof relay |06 connects, through its front contacts, `transmitting line 14to channel 24 so that the speech energy after traversing delay device|03 is sent out over channel 24. Operation of relay |05 interrupts atits back contacts ther connection of transmitting line 81 of subscriberset I4 to vline |01 and connects, at its front contacts, transmit tingline 81 to line I I I. f

The speech energy passing out over channel 24 is received by subscriberE--I of the east ter- As the .east terminal equipment has not beenillustrated let us refer for the moment to the receiving equipment ofthe west terminal and assume that speech energy is being received fromthe distant terminal over channel 25. Low-pass filter II2 preventspassage of the high frequency voice currents into path lI I3 ,so thatvrelay I I4 remains in normal, unoperated position. The speech energy,therefore, passes through the back contacts of relay I I4 to receivingline 15 and over this line to subscriber set I I.

Returning now to consideration of the transmittingportions of thecircuit, let us assume that with v subscriber W-I still talking,,subscriber W-4 also starts talking, and that Aneither :sub-

called that relays |05 and |06 are in yoperated position due to the factthat subscriber W-I is talking.

Speech energy `produced by subscriber set vI4 is applied to transmittingline 81 vthrough transformer |02, a part of the energy passing throughamplifier-detector H and causing operation of relay H6. Operation ofrelay H6 completes a path through its front contacts over which a lowfrequency tone produced by source 43 is applied to line 81. We now haveon line 81, therefore, both the speech currents of voice frequency andthe characteristic toney of low frequency. High-pass filter I I1 servesto exclude the low frequency tone currents from the operating circuit ofrelay I|6.

Relay |05 being in operated position, as set forth above, the voicefrequency currents and tone signals impressed on line 81 are appliedthrough the front contacts of relay |05 to line III and in turn throughthe back contacts of vrelay |32, which is in unoperated position, to

line |33. As line |33 is connected to channel 26, the voice frequencysignals and the tone signals originating at subscriber station I4 arenow impressed on channel 26 for transmission to the east terminal.

The currents impressed on channel 26 are received by subscriber E-4 atthe east terminal. As the equipment of the east terminal has not beenillustrated let us assume for the moment that voice frequency currentsand the charac-v teristic low frequency tone are being received overchannel 21. The tone signals are of a suiiiciently low frequency to bepassed by low-pass lter |34 and are, therefore, impressed onamplifier-detector |35. The energy emanating from amplifier-detector |35causes operation of relay |36. Operation of relay |36 is effective tointerrupt at its back contacts the connection of channel 21 andreceiving line 82 and to complete at its front contacts the connectionof channel 21 to receiving line IOI. The voice frequency currentsincoming over channel 21 are received therefore by subscriber set I4. Ifdesired, a highpass lter may be included in the receiving path in orderto exclude any part of the tone signals therefrom.

Returning now to consideration of the transmitting end of the system, itis believed that it will be clear from the above how the arrangementwill function with'other combinations of talkers. For example, under theconditionsv described above, i. e., with W-I and W-4 talking, shouldsubscriber W-2 also start talking, the trans- 'mitting path ofsubscriber W-4 will be transferred from channel 26 to channel 4| byoperation of relay |32, relay |31 being in unoperated position assubscriber W-3 is not talking. However, should all three of thepreferred subscribers, W-I, W-2 and W--3, be talking, transmission fromW-4 will be delayed until an idle channel is available.

Considering again the situation iirst described, i. e., with subscribersW-I and W-4 talking, should subscriber W-I stop talking relays |05 and|06 will drop back to unoperated position and transmitting line 81 ofsubscriber W-4 will now be connected over line |01 to channel 24 and theAvoice frequency currents and the tone signals Will reachthe eastterminal over channel 24, In order to make clear the action of thereceiving equipment let us assume that the voice frequency currents andtone signals produced by the subscriber set corresponding to set I4 arenow received over channel 25. The low frequency tone currents are passedby low-pass filter H2 and impressed on amplifier-detector |4I. Theenergyl emanating from amplifier-detector I4! causes operation of relayH4 which results, in turn, in the transfer of channel from receivingline 15 to receiving line |0I. The voice frequency currents are receivedtherefore by subscriber set I4.

It is believed it will be clear from the above that the speech currentsand tone signals originating at subscriber set I4 are impressed onchannel 24 if subscriber W-I is not talking; on channel 26 if subscriberW-I is talking and subscriber W-Z is idle; on channel 4I `if subscribersW-I and W-2 are talking and subscriber W-3 is idle; and thattransmission from set I4 is delayed in the event that all threepreferred subscribers are talking. It will be clear also, with respectto the receiving arrangement, that incoming channel 42 is normallyconnected to the receiving equipment of subscriber set |3, channel 21 isnormally connected to the receiving equipment of subscriber set I2 andchannel 25 is normally connected to the receiving equipment ofsubscriber set II. In the event, however, that the low frequency toneoriginating at the fourth or switched subscriber set is present on achannel, said channel is switched to the receiving equipment ofsubscriber set |4 which'is the fourth or switched set of the Westterminal.

While a specific embodiment of the invention has been selected fordetailed description it will be understood that the invention is not`limited in its application to the embodiment described. It will beapparent, for example, that otherl combinations of subscriber sets maybe provided. For example, one switched subscriber set may be associatedwith a larger number of preferred subscriber sets than the threeillustrated. 'I'he embodiment described should be taken as illustrativeof the invention and not as restrictive thereof. f

What is claimed is:

'1. In a speech transmission system including an east terminal and awest terminal, a plurality of transmitters at the east terminal, acorresponding number of receivers at the West terminal, a correspondingnumber of speech transmission channels connecting said east terminal andsaid west terminal, means operated by energy caused by production of aspeech fragment of vocal interval duration by one of said transmittersfor connecting the output of said transmitter to the speech transmissionchannel associated therewith, an additional transmitter at said eastterminal, a corresponding additional receiver at said west terminal,means controlled by voice energy vfor connecting the output of saidadditional transmitter to the transmitting end of one of said speechtransmission channels, and means controlled over said last-mentionedchannel for connecting said additional receiver to the receiving end ofthe same channel.

2. In a speech transmission system including an east terminal and a westterminal, a, plurality of transmitters at the east terminal, acorresponding number of receivers at the west terminal, a correspondingnumber of speech transmission channels connecting said east terminal andsaid west terminal, means effective in response to production of aspeech fragment of vocal interval duration byl one of said transmittersfor connecting the output of said transmitter to a predetermined oneof'said speech transmission channels, an additional transmitter at saideast terminal, a distinctive frequency tone source at said eastterminal, an additional receiver'at said west terminal, means operatedby energy caused by production of a speech frag'- tional' transmitterfor connecting the outputs of said transmitter and of said tone sourcetothe transmitting end of one of said speech transmission channels whichis then idle so far as transmission by said plurality of transmitters isconcerned, and means controlled by said distinctive tone for connectingsaid additional receiver to the receiving end of the same channel.

3. In a speech transmission system including an east terminal and a westterminal, a plurality of transmitters at the east terminal, acorresponding number of receivers at the west terminal, a correspondingnumber of speech transmission channels connecting said east terminal andsaid west terminal, means operated by energy caused by production of aspeech fragment of vocal interval duration by one of said transmittersfor connecting the output of said transmitter to one of said speechtransmission channels, an additional transmitter at said east terminal,means for connecting the output of said additional transmitter to one ofsaid channels not in use at the time, and means operated by energyresulting from application to said last mentioned channel of a speechfragment of vocal interval duration by one of said plurality oftransmitters for transferring the output of said additional transmitterto another of said channels not in use at the time.

4. In a speech transmission system including an east terminal and a westterminal, a first transmitter, a `second transmitter and a thirdtransmitter at said east terminal, corresponding `first, second andthird receivers at said west terminal, a first speech transmissionchannel between said first transmitter and said first receiver, a secondspeech transmission channel between said second transmitter and saidsecond receiver, and 'a third speech transmission channel between saidthird transmitter and said third receiver, a fourth transmitter at saideast terminal, a fourth receiver at said west terminal, means forconnecting the output of said fourth transmitter to said rst channel,and means operated by energy caused by production of a speech fragmentof vocal interval duration by said first transmitter for connecting theoutput of Said first transmitter to said first channel and fortransferring the output of said fourth transmitter to said secondchannel.

5. In a speech transmission system including an east terminal and a westterminal, a first transmitter, a second transmitter and a thirdtransmitter at said east terminal, corresponding first, second and thirdreceivers at said west terminal, a first speech transmission channelbetween said rst transmitter and said first receiver, a second speechtransmission channel between said second transmitter. and said secondreceiver and a third speech transmission channel between said thirdtransmitter and said third receiver, a fourth transmitter at said eastterminal, a fourth receiver at said west terminal, means for connectingthe output of said fourth transmitter to said first channel, meansoperated by energy caused by production of a speech fragment of vocalinterval duration by said first transmitter for connecting the output ofsaid `first transmitter to said first channel and for transferring theoutput of `said fourth transmitter to said second channel, and meansoperated by energy caused by production of speech fragments of vocalinterv-al duration by both said first and said second transmitters fortransferment of *vocal interval duration by -said .addiring the outputof said fourth transmitter to ,said third channel.

6. In a speech transmission system including `an east terminal and awest terminal, a first transmitter, a second transmitter and a thirdtransmitter at said east terminal, corresponding first, second and thirdreceivers at said West -terminal, a first speech transmission channelbetween said iirst transmitter and said first receiver, a second speech`transmission channel between said second transmitter and said secondreceiver and a third speech transmission channel between said thirdtransmitter and said third receiver, a fourth transmitter at said eastterminal, a fourth receiver at said `west terminal, means for connectingthe output of Said fourth transmitter to said first channel, meansoperated by energy caused by production of a speech fragment of vocalinterval duration by said first transmitter for connecting the output ofsaid first transmitter to said first channel and for transferring theoutput of said fourth transmitter to said second channel, a distinctivefrequency tone source at said east terminal, means operated by energycaused by production of a speech fragment ofivocal interv-al duration bysaid fourthl transmitter for connecting the output of said tone sourceto that channel to which the output of said fourth transmitter isconnected and means controlled by said distinctive frequency tone forconnecting said fourth receiver to the receiving end of the channel towhich the output of said fourth transmitter is connected.

'7. In a speech transmission system including an east terminal and awest terminal, a first transmitter, a second transmitter and a lthirdtransmitter at said east terminal, corresponding first, second and thirdreceivers at said west terminal, a first speech transmission channelbetween said first transmitter and said first receiver, a second speechtransmission channel between said second transmitter and said secondreceiver, and a third speech transmission channel between said thirdtransmitter and said third receiver, a fourth transmitter at said eastterminal, a fourth receiver at said west terminal, means for connectingthe output of said fourth transmitter to said rst channel if idle, andswitching means operated by energy caused by production of a speechfragment of Vocal inter- `val duration by said first transmitter totransfer the output of said fourth ltransmitter to another of saidchannels.

8. In a speech transmission system including an east terminal and a Westterminal, a .first transmitter, a second transmitter and a thirdtransmitter at said east terminal, corresponding first, second and thirdreceivers at said west terminal, a first speech transmission channel"between said first transmitter and said first receiver, a second speechtransmission channel between said second transmitter and said secondreceiver, and a third speech transmission channel between said thirdtransmitter and said third receiver, a fourth transmitter at said eastterminal, a fourth receiver at said west terminal, means for connectingthe output 'of said fourth transmitter to said first channel, switchingmeans operated by energy caused by production of a speech fragment ofvocal interval duration by any one of said first three transmitters topi'event connection of the output of lsaid fourth transmitter to thatchannel which is between the speech producing transmitter and thecorresponding receiver.

9. In a speech transmission system providing between stations aplurality of transmission paths over a like number of channelsdefinitely assigned for use by said respective stations. the method ofproviding for an additional transmission path which comprises the stepof switching the additional transmission path about among said channelsat intervals when said channels are not being used for transmission bysaid stations.

10. In a speech transmission system providing between stations aplurality of transmission paths over a plurality of westward channelsand a like number of eastward channels, said channels being definitelyassigned for use by said respective stations, the method of providingfor an additional transmission path in each direction which comprisesthe step of independently switching the additional westward path and theadditional eastward path about among said respective westward channelsand said respective eastward channels at intervals when said chanvnelsare not being used for transmission by said

